Polycarbonates are well known, commercially important materials which are produced in large quantities. Such polymers are typically prepared by reacting a carbonate precursor with a dihydric phenol to provide a linear polymer consisting of units of the dihydric phenol linked to one another through carbonate linkages. These polycarbonates have outstanding mechanical, thermal and optical clarity (transparency), thermal and dimensional stability and impact strength. However, such polycarbonates typically differ from most thermoplastic polymers in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. However, in contrast to most thermoplastic polymers, polycarbonates prepared from dihydric phenols typically exhibit Newtonian flow at normal processing temperatures.
Two other characteristics of molten thermoplastic polymers are considered to be significant for molding operations: melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt from distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and indicates the ability of the melt to support a stress. Both of these characteristics are important in extrusion blow molding, particularly in fabrication by extrusion blow molding. Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to polymers thus allowing their use in blow molding fabrication. In the usual blow molding operation, a tube of molten thermoplastic is extruded vertically downward into a mold, followed by the introduction of a gas, such as air, into the tube thus forcing the molten plastic to conform to the shape of the mold. The length of the tube and the quantity of material forming the tube are limiting factors in determining the size and wall thickness of the objects that can be molded by this process. The fluidity of the melt obtained from bisphenol-A polycarbonate, or the lack of melt strength as well as the paucity of extrudate swelling, serve to limit blow molding applications to relatively small, thin-walled parts. Consequently, the Newtonian behavior of polycarbonate resin melts severely restricts their use in the production of large hollow bodies by convention extrusion blowmolding operations as well as the production of various other shapes by profile extrusion methods.
Thermoplastic, randomly branched polycarbonates exhibit properties of non-Newtonian flow, melt elasticity and melt strength which permit them to obtain such articles as bottles which were not heretofore easily or readily produced with linear polycarbonates.
U.S. Pat. No. 4,001,184, issued Jan. 4, 1977, discloses preparing thermoplastic, randomly branched polycarbonates by reacting a dihydric phenol, a carbonate precursor and a polyfunctional aromatic compound which contains at least three functional groups selected from the group consisting of carboxyl, carboxylic anhydride and haloformyl, or mixtures thereof.
In this process, at least 20 to 100 weight percent of the stoichiometrically required amount of the carbonate precursor must be added to the reaction medium while said medium is maintained at a pH of about 3 to about 6. The pH of the mixture is thereafter raised to at least about 9 but less than about 12, at which time the remaining portion, if any, of the carbonate precursor is added to the reaction medium. While this process produces branched polycarbonate resins of high quality, the additional production steps and process monitoring necessitated by the pH processing profile of the reaction medium are disadvantageous for large scale commercial operations.